Leak Risks & Hidden Damage Risks in Hot roof designs, aka dense-packed insulated sloped roofs:
This article describes the causes of & risks of leaks into enclosed, un-vented, inaccessible or dense-packed roof cavities or cathedral ceilings. There, undiscovered for a time because of the very nature of the construction, leaks into a hot roof risk costly mold, & structural damage.
This article series about roof and ceiling ventilation describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems in buildings.
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Our page top photo, which is not of a current "hot roof" or "dense packed" insulated roof, shows the sort of rot that can occur when a roof cavity, or for that matter an attic, is not accessible for inspection and when leaks go unattended. There are two principal moisture pathways into a "packed" sloped roof
Watch out: when enthusiasts recount differences between the performance of a "packed" or "hot roof" design (presumably very high-R and "tight") and a ventilated roof design (more trouble, more cost, sometimes poorer insulation and higher energy cost) they sometimes may not have realized, or may fail to report that when a leak into the packed roof occurs, water stays there a long time - inviting rot, carpenter ants, mold.
The result is a very large repair expense.
At the Journal of Light Construction building conference in Boston way back in 1985 we heard from the hot roof camp, we heard research reporting that most moisture leaking into (and thus potentially wetting) building cavities was through penetrations in the interior ceilings such as around light fixtures.
Following those talks, we saw slides of horror stories by a roofer, Henri De Marne's, whose business was going to New England's hot-roofed cathedral-ceiling homes where a leak that occurred from above (damaged roof shingles) had gone unnoticed long enough for the repair to require more than just fixing a leak.
The repairs for the cases he documented required a complete roof tear-off, roof sheathing removal, wet insulation removal, moldy drywall removal, replacing rotted rafters, and then finally, reconstruction.
It would appear that
We [DJF] recently constructed a "hot roof" designed second floor addition to serve as an office, using sprayed foam insulation in the addition's ceiling, walls and floor for a super-tight, super-insulated building.
The combination of relatively low slope roof and cathedral ceiling design argued for a well-insulated hot-roof design. We used Demilec™ open celled spray foam as shown at below left. We describe this insulation project and material
at FOAM SPRAY INSULATION TYPES. The trouble began several winters later at a leak point shown in the second photo at below right.
However, considering my field experience with the very high costs of leaks into un-vented hot roofs, we took the extra precaution of installing the most-durable and damage resistant roof we could find: a heavy-gauge standing seam metal roof.
See METAL ROOFING
Watch out: Even with this very "safe" metal roof over our un-vented roof cavity we had trouble. It was my own fault for deferring the installation of snow guards (the building is in New York state). After just a few years of trouble free existence, heavy snows broke off a plumbing vent causing a leak into the roof cavity.
See SNOW GUARD FAILURES
For details about how we repaired this mess see PLUMBING VENT REPAIR.
We caught this leak in time (thanks to annual on-roof inspections), and we were able to open and dry the roof cavity from above before repairing the plumbing vent ... and installing the missing snow guards.
See SNOW GUARD GLUE ON INSTALLATION (we are testing both glue-on and clamp-on snow guards on this roof)
Late last year you and I had a wonderful dialog via email about a roof venting situation at my house (I have pasted the thread below). I am most grateful for your advice.
[See ROOF VENTILATION IMPROVEMENTS for that discussion that took place in 2013- Ed. ]
[Click to enlarge any image]
I ended up going for a "hot roof" and using the great subsidized weatherization program which installed 6" fiberglass batt on the kneewall slope and covered it with 2" thermal barrier polyiso.
So I was happy with what they did except the prior owners goofs came back to haunt me last night with the torrential rain we had. If you recall my house has a fairly new shed dormer which the prior owners installed with a shoddy and shady contractor.
Last night [9 Dec 2014] with all the rain the roof sprang a leak which came through one of the recessed lighting fixtures beneath the ridge of the roof (not near the kneewall slopes). I got my renovation contractor to send his roof guy and he took the photos (attached from the roof.
The roof guy said the shed dormer roof is rubber and the rest is asphalt. He said he couldn't do anything now because nothing sticks to rubber when it's wet.
But basically his assessment is that the leak is as a result of the wind blowing rain sideways into the stupid ridge vent the prior owners put in AFTER the shed dormer was completed. They put it in because the town inspector told them to, even though it doesn't have the "proper" or "baffle" vents which would create a ventilation channel to the soffits (it's packed with fiberglass insulation under the roof). So there's no point to the ridge vent (as we discussed in the thread below).
So, the roof guy that perhaps the best solution would be to cap off the ridge with metal.
Would you be willing to take a quick look at the pictures when you get a chance and let me know what you think I should do to (hopefully) stop the leak? Would capping the ridge vent create further problems? Any other suggestions? - A.G. 10 Dec 2014
Installing a ridge vent where one should not have been placed - on a sealed-cavity, un-vented, "hot roof" design building was in my opinion a fundamental error that risked compromising the building design and that could indeed cause costly water damage. Further, as installed and shown in your photo below, there is no working "ridge vent" that I can see.
All I see are cap shingles over a ridge that marks the peak of a roof that is lower slope on one side. But if a ridge vent "slot" was cut into the roof deck below the ridge cap shingles wind-blown rain could certainly leak into the structure.
In an otherwise un-vented hot-roof design, an exit vent at the ridge will not function to ventilate the roof, even if it does not cause a leak. In sum we have accomplished nothing except providing a way for the roof to leak.
Your report of water leaking through a recessed light fixture in the ceiling below confirms that worry.
From the photos you submitted, unless there was some other roof area that is penetrated by anything that thus could have leaked, it is indeed apparent that wind-blown rain swept up-roof and entered at the un-sealed "ridge vent" opening.
Removing, capping, or completely sealing the ridge vent against water entry is, I am sorry to say, the easy part of the repairs that your home requires now.
In my experience unless you properly open, clean and dry the roof cavity, troublesome, costly, and possibly even unhealthy mold contamination is very likely even if no further leaks ensue. A single soaking of a fiberglass-insulated, drywall-covered building ceiling or wall cavity is enough to make mold contamination likely in that area.
Step 1: Seal the roof against further water entry. Your roofer may choose to remove the ridge vent and continue other membrane or metal roofing up over the ridge. Be sure that the new seal is reliable against leaks including wind-blown rain from any direction.
Step 2: Test cuts to find the extent of water entry: We need to find and remove all wet materials in the roof cavity or ceiling below. Make a series of test cuts into the ceiling below, starting at the area that you know was leaking - up-roof and also down-roof from the pot light from which you report water leaking. You can make test cuts as small as a 2" x 4" but I suspect that water entered the full width of the roof cavity beginning at the ridge, given the roof design and placement of the ridge vent.
Just how far down in the roof cavity water penetrated depends on several factors including the tightness of packing of insulation, the roof slope, variations in water entry as ridge vent openings may have varied, and the total quantity of water that entered the cavity.
You may have to make strip cuts across the full width of the roof to permit an adequate inspection. A strip cut is made through the drywall, from wall to wall, about 12" wide, to permit inspection of every rafter bay for water entry. You may need more than one of these, one above the light fixture from which water leaked, and possibly a lower one down-slope depending on what you find around the light fixture and in other rafter bays.
Step 3: Remove all wet or moldy materials:
Watch out: typically we have 24-48 hours to remove wet drywall and insulation if we are to avoid a more costly mold cleanup-job. Don't panic, but do clean properly so as to avoid more costly cleanup expenses later.
You need to discover the full extent of what got wet and then to remove and discard, dry and clean the roof cavity, after removing:
Step 4: Completely dry the roof cavity that has been exposed before beginning further restoration or "put back" of insulation or drywall. If it was necessary to clean moldy framing or roof sheathing you may want the added insurance of applying a fungicidal sealant.
Step 5: Test the roof seal while the ceiling below is still open for inspection: When the roof cavity is fully dry and before replacing any materials you might want to try spraying water up-roof towards the now-sealed ridge to be double sure that there are no remaining leak openings. This step may prevent you from having to make this disruptive repair all over again.
Replace insulation & drywall: When the roof cavity is clean and dry and not leaking, replace insulation, any vapor barriers, and drywall.
Step 6: Seal any ceiling penetrations on the interior of the building: Be sure also to seal around any ceiling penetrations to avoid leaking moisture-laden air into the roof cavity from the building interior - another common source of mold contamination in sealed roof or other roof cavities.
I would like to see photos as well as to read reports of what you discover during the investigation for moisture and mold in the roof cavity. That information may permit further comment.
Watch out for real world snafus, damage, leaks in roofs. In sum, I'm left unsure about the gap between new construction designs and a perfect world where roofs never leak and the roofs I and more importantly, repair and renovation roof contractors have found when we inspected, tore apart, and repaired leaky roofs of both insulated cathedral ceiling homes, and homes with vented roof cavities.
Our expert Steven Bliss commented to offer a final word on Hot Roof Designs:
I agree with you that, in the real world, this is not such a good idea. If there's a flashing leak or other roof leak, you could have a pretty soggy mess that stays wet for a long time and could cause structural decay.
Plenty of people are building hot roofs, but I wouldn't -- except maybe one with spray urethane which won't absorb much water like cellulose would. 
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